Responsive React design relies on standard CSS responsive techniques applied through React's component structure. You have three main paths: traditional CSS/SCSS with media queries imported into components, CSS-in-JS solutions like styled-components that let you write breakpoint logic inline, or utility frameworks like Tailwind CSS that provide responsive classes out of the box. The cleanest approach for most projects combines CSS Grid or Flexbox layouts with relative units (rem for typography, percentages or viewport units for containers) and strategic media queries at common breakpoints: 640px (mobile), 768px (tablet), 1024px (desktop), and 1280px+ (wide screens). In React, you typically define these in component-level stylesheets or styled-components, keeping styles co-located with the components they affect. For dynamic behavior based on screen size, use the window.matchMedia API or libraries like react-responsive to conditionally render components or alter props. A custom hook wrapping matchMedia gives you clean boolean flags like isMobile or isTablet throughout your app. This matters when you need fundamentally different UI patterns, not just layout shifts—like showing a drawer menu on mobile versus a horizontal nav on desktop. At Ottawa SEO, we default to Tailwind CSS for React projects because its responsive modifiers (sm:, md:, lg:) let developers see breakpoint behavior directly in JSX without context switching to CSS files. This speeds up development and keeps responsive logic explicit. For complex animations or theme-dependent styling, we layer in styled-components. Key mistakes to avoid: hardcoding pixel widths instead of using flexible units, testing only on your laptop screen, and over-relying on JavaScript for layout that CSS handles better. Always test on actual devices—Chrome DevTools is useful but doesn't catch every rendering quirk. Image optimization matters too: use srcset and modern formats (WebP, AVIF) with React Image components or Next.js Image to serve appropriately sized assets per viewport. A 2000px hero image costs mobile users load time and data they shouldn't pay for.